Data integrity and availability is a critical factor in large computer data systems. Consequently, backup data storage systems have been developed to prevent the loss of data in the event of various types of failures. One such backup system is known as “peer-to-peer remote copy” (PPRC). As illustrated in FIG. 1, in a PPRC system 100, data generated by a host device 110 is stored on a primary storage unit 120. A copy of the data is also transmitted, such as over a fibre channel network 130, and stored on a secondary storage unit 140. Because of the flexibility of network interconnections, the primary and secondary units 120 and 140 may be physically located remote from the host 110. And, for data security, the primary and secondary units 120 and 140 are physically located distant from each other, thereby reducing the likelihood of a single disaster simultaneously harming both the primary and secondary units 120 and 140.
The distance by which the primary and secondary units 120 and 140 may be separated is dependent upon numerous factors. One significant factor is the total response time of each I/O operation (such as a write operation); that is, the amount of time required for a block of data to be transferred from the primary storage unit 120 to the secondary storage unit 140, including all handshaking. Typically, the longer the response time, the shorter the distance which may practically separate the two units. And, a significant factor in determining the response time is the number of round trips of command and data which must take place to complete a transfer of data. As will be appreciated, the more round trips which are necessary, the slower the effective transfer rate becomes.
One such round trip occurs when the primary and secondary units 120 and 140 exchange “transfer ready” signals prior to a write operation. The primary unit 120 (also known as the initiator) transmits a message to the secondary unit 140 (also known as the target) indicating that data is ready to be transferred. Until the primary unit 120 receives an appropriate acknowledgement from the secondary unit 140, transfer of the data cannot begin. Among other items, the acknowledgement indicates that the secondary unit has prepared the necessary buffers and is ready to receive the data. Such preparation may entail some delay and the handshaking itself results in some delay as well. Thus, such a transfer requires two round trips (the transfer ready exchange and the transfer of data with a subsequent acknowledgement of receipt) and results in a corresponding delay.
Another round trip occurs when additional control information is transferred from the primary unit 120 to the secondary unit 140 before the data itself is transferred. Such additional control information may not be able to fit within a conventional write command, such as a command descriptor block (CDB). Thus, another round trip is necessary to separately transfer the control information.
Other factors may necessitate further round trips. Transferring data over a conventional fibre channel network may entail three to four round trips and the distance between the primary storage unit 120 and the secondary storage unit 140 may thus be limited to about 100 kilometers.
Consequently, in order to increase the distance between the primary and secondary units, it remains desirable to reduce the response time for data transfers.